This work describes the integration of Schottky diodes into fast GaN MMIC process technology suitable for the realization of switch-mode power amplifier core chips for class-S operation at 2 GHz. With the demonstration of this technology, the so-called third-quadrant issue, which reduces the efficiency in band pass-Δ-Σ class-S operation can be diminished on device level. Compared to a hybrid diode assembly, the broadband properties of the amplifier module with on-chip-integrated diode can be improved by the reduction of parasitic losses. The GaN heterostructure field effect transistors (HFETs) with integrated series diode show a cut-off frequency of 28 GHz with drain breakdown voltages exceeding −100 and +100 V and comparable large signal performance to conventional GaN HFETs at 10 GHz. MMIC core chips for class-D and class-S switch-mode power amplifier modules are demonstrated for the operation at mobile communication frequencies between 0.45 and 2 GHz and signal bit rates up to 8 Gbps. The circuits yield broadband output power levels between 4 and 9 W with efficiencies of up to 80%.

This paper reports on a novel voltage-mode class-S power amplifier for the 450 MHz band, based on GaN–HEMT monolithic microwave integrated circuits (MMICs). It achieves a peak output power of 3.4 W for a single tone at 400 MHz, encoded in standard band-pass delta-sigma modulation with 1.6 Gbps sampling frequency. The corresponding efficiency is 38%, peaking at 52% for 0.5 W output power. In order to demonstrate the influence of coding efficiency, additional measurements using a periodic square-wave signal were performed (class-D operation), which yield a maximum output power of 7 W with 64% efficiency. To the author's knowledge, these are the best results in this frequency range achieved so far with the voltage-mode class-S configuration. The paper discusses behavior at power back-off and the influence of the loss mechanisms.